In optical data transmission, a signal to be transmitted is sent as a sequence of light pulses over an optical fiber to a photo detector which converts the optical signal into an electronic one for subsequent processing. The most straightforward method of data transmission is to provide a different optical fiber per transmission. However, the use of a different fiber per transmission is expensive and therefore various techniques were proposed to allow multiple signals to be transmitted over a single fiber. The two most common techniques are Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM).
In TDM, separate input signals are carried on a single fiber by allocating time transmission windows. The input signals are fed to a multiplexer which schedules use of the optical fiber so that each input signal is allowed to use the fiber in a specific time slot. At the receiver, synchronization techniques are used to ensure that the different input signals are sent on to the appropriate destination.
In WDM, the fiber is shared by sending each input signal at the same time, but on a different carrier wavelength channel, for example a first signal could be transmitted using a carrier wavelength of 1539 nm and another signal is transmitted using a carrier signal of 1560 nm.
All modern optical data transmission utilizes fiber TDM, with core transmission additionally utilizing WDM. In core data transmission, individual signals rates of up to 100 Gbit/sec are achieved through the use of TDM; these individual signals are then multiplexed onto a signal fiber through WDM in order to further enhance the transmission rate.
Considering WDM in greater detail, a grid of wavelengths is specified by the International Telecommunication Union (ITU) so that compliant equipment from different manufacturers can operate together. The ITU has specified a number of Dense Wavelength Division Multiplexing grid sizes at 12.5 Ghz, 25 Ghz, 50 Ghz and 100 Ghz. 50 Ghz is currently the most popular channel and, using the DP-QPSK modulation format, it is possible to fit a 100 Gbit/s signal within a single channel in the 50 Ghz grid. However, research into optical transmission beyond 100 Gbit/s has shown that higher spectral efficiency formats have to be used, or the spectral width of the signals must be increased to support 400 Gbit/s or 1 Tbit/s transmission. Utilizing modulation formats with higher spectral efficiencies limits the distance the signal can propagate due to OSNR penalties, and increasing the spectral width means that the signal can no longer fit within the widely deployed 50 Ghz ITU grid. To overcome these problems, flexible grid or Flexgrid networks have been proposed. In this scheme, arbitrary sized wavelength blocks can be specified by the network owner, thereby accommodating new bit rate services.
In order to transmit signals by WDM, whether on the fixed grid or flexible grid network, two signals having different carrier wavelengths must be multiplexed onto the same line. Providing the carrier wavelengths are sufficiently different, the signals will not interfere with each other. At the end of the optical fiber, the incoming light signals are demultiplexed into the individual signals, which are subsequently processed as required.
Current telecommunications networks comprise a single optical fiber for data transmission in a given direction. The nodes at which these fibers meet are classified according to the number of fiber directions that converge at that node. For example, if optical fibers deliver data to and from North, South and West then the node at which these fibers meet is a degree three node. It will be appreciated that six fibers converge at a degree node if the network comprises a single fiber per direction: one fiber for data transmission from North, one fiber for data transmission to North, etc.
However, due to the ever increasing bandwidth demands on telecommunications networks, it is anticipated that multiple fibers per direction will be required in the near future. Accordingly, many more fibers will converge at a node of a given degree. For example, a degree three node in a “multi-fiber” network may comprise six or more fibers. In a multi-fiber arrangement such as this, it is envisaged that a number of independent channels or superchannels will be spread across the multiple fibers, the number of channels or superchannels carried on any one of the fibers being variable in accordance with the optical spectrum and/or the network architecture.
One known device for demultiplexing WDM signals is a grating demultiplexer, which operates on the principle of light dispersion: as an optical signal is passed through a grating demultiplexer, the various wavelengths contained within that signal are deflected by varying angles. The grating therefore acts to break down the optical signal into its constituent wavelength spectrum, which enables certain wavelength channels within that spectrum to be isolated and subsequently processed as required. Grating demultiplexers work moderately well with the fixed grid network, providing there are a low number of input fibers. However, there are likely to be problems associated with the use of grating demultiplexers in the flexible grid network and/or for large numbers of input fibers. One problem is that grating demultiplexers demultiplex at fixed 50 GHz outputs and are therefore incompatible with Flexgrid. Another problem is that grating demultiplexers are directional: the direction at which a certain wavelength channel is output from a grating demultiplexer is a function of the wavelength thereof. This is particularly problematic in a “multi-fiber” network since it is not possible to direct a single wavelength channel carried on multiple fibers to a single spatial location. Yet another problem is a lack of flexibility: the only way in which to change the wavelength channel(s) received by particular receivers is to physically move the equipment at the exchange.